doc: Fix image use in PDF build with vpath
In a vpath build, we need to point to the source directory to allow
FOP to find the images.
Branch
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REL_12_STABLE
Details
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https://git.postgresql.org/pg/commitdiff/842ac79407a1368308beff230d3e83aad6d82b75
Modified Files
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doc/src/sgml/Makefile | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)